Some materials, both organic and metallic, have shape memory. An article made of such materials when deformed “remembers” its original, cold-forged shape, and returns to its pre-deformed shape when heated. The three main shape memory alloys are copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titanium (NiTi). NiTi shape memory alloys have two different temperature-dependent crystal structures (phases) called “martensite” (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different.
When heated, martensite NiTi begins to transform into austenite at a temperature called the austenite start temperature (As), and completes the transformation at a temperature called the austenite finish temperature (Af). When cooled, austenite NiTi begins to transform into martensite at a temperature that is called the martensite start temperature (Ms), and is again completely reverted at a temperature called the martensite finish temperature (Mf).
Composition and metallurgical treatments have dramatic impacts on the above-mentioned transition temperatures. For practical applications, NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easily deformed (somewhat like soft pewter). Superelastic NiTi is highly elastic (rubber-like), while austenitic NiTi is quite strong and hard (similar to titanium).
NiTi has all of these properties, and their specific expression depends on the temperature in which the NiTi is used.
The temperature range for the martensite-to-austenite transformation, i.e., soft-to-hard transition, that takes place upon heating is somewhat higher than that for the reverse transformation upon cooling. The difference between the transition temperatures upon heating and cooling is called hysteresis (denoted as H). Hysteresis is generally defined as the difference between the temperatures at which the material is 50% transformed to austenite upon heating and 50% transformed to martensite upon cooling. This difference can be up to 20-30 degrees C. In practice, this means that an alloy designed to be completely transformed by body temperature upon heating (Af<37 degrees C.) would require cooling to about +5 degrees C. to fully retransform into martensite (Mf).
One of the commercial uses of shape memory alloy exploits the pseudo-elastic properties of the metal during the high-temperature (austenitic) phase. This is the result of pseudoelasticity; the martensitic phase is generated by stressing the metal in the austenitic state and this martensite phase is capable of large strains. With the removal of the load, the martensite transforms back into the austenite phase and resumes its original shape. This allows the metal to be bent, twisted and pulled, before reforming its shape when released. This means the frames of shape memory alloy glasses are claimed to be “nearly indestructible” because it appears no amount of bending results in permanent plastic deformation.
The martensite temperature of shape memory alloys is dependent on a number of factors including alloy chemistry. Shape memory alloys with transformation temperatures in the range of 60-1450 K have been made.
Many shape memory alloys (SMAs) are known to display stress-induced martensite (SIM). When an SMA sample exhibiting stress-induced martensite is stressed at a temperature above Ms (so that the austenitic state is initially stable), but below Md (the maximum temperature at which martensite formation can occur even under stress) it first deforms elastically and then, at a critical stress, begins to transform by the formation of stress-induced martensite. Depending on whether the temperature is above or below As, the behavior when the deforming stress is released differs. If the temperature is below As, the stress-induced martensite is stable; but if the temperature is above As, the martensite is unstable and transforms back to austenite, with the sample returning (or attempting to return) to its original shape. The effect is seen in almost all alloys which exhibit a thermoelastic martensitic transformation, along with the shape memory effect. However, the extent of the temperature range over which SIM is seen and the stress and strain ranges for the effect vary greatly with the alloy.
Ryhänen J, in “Biocompatibility evaluation of nickel-titanium shape memory metal alloy,” Academic Dissertation, Faculty of Medicine, Department of Surgery, University of Oulu, Finland (May 1999), which is incorporated herein by reference, describes the shape memory effect, superelasticity, and good damping properties that make the nickel-titanium shape memory metal alloy (Nitinol or NiTi) a fascinating material for surgical applications. Among other things, the dissertation describes the mechanical properties of NiTi in Section 2.3.8, including Table I thereof.